Surveillance and identification of target radio signals in the dynamically changing RF spectral landscape requires various broadly-tunable RF filters. In view of potential jamming noises, RF notch filters with narrow resolution bandwidth and high extinction are desired to recover the signals of interest with high fidelity.
Photonics-enabled RF filters are promising since provide potentially wider tunability and re-configurability in comparison to traditional electronic filters. Photonics-enabled RF filters also exhibit improved immunity to electromagnetic interference (EMI) over traditional electronic filters. Sustained efforts in the past decades for developing RF photonic filters for military applications have demonstrated significant benefits, such as low loss, wideband tunability and immunity to EMI.
However, almost all conventional RF filters employ discrete fiber optical components resulting in size, weight and power (SWAP) characteristics that are not consistent with operating in constrained environments. More recent efforts in developing chip-scale RF photonics have produced devices with much smaller sizes than their fiber counterparts but at the cost of lower performance. For example, chip-scale optical ring resonator filters have exhibited bandwidths as low as about 200 MHz. While this may be sufficient for channelizer applications, it is undesirable for a notch filter.
Achieving large extinction in addition to high Q in a ring resonator poses additional challenges. High extinction using ring resonators can be achieved by accomplishing critical coupling, where the energy dissipation in the ring is equal to the net coupling losses. Power coupling ratios are mainly controlled by the gap between the ring waveguide and the bus waveguides. However, process variation may not allow reproducible fabrication of a target structure. While tunable coupler structures may assist with reproducible fabrication, it introduces excess loss limiting a high Q factor.